The ultraviolet (UV) wavelengths of sunlight can cause sunburn (erythema) and blistering (edema). Exposure to ultraviolet light can also cause the skin to feel dry and taut in moderate doses, and to peel if exposed to higher doses. However, there are also more subtle acute effects that are not as readily discernable, such as photo-immunosuppression, cross-linking of deoxyribonucleic acid (DNA), formation of sunburn cells, and loss of Langerhans cells. Even more serious long term effects can occur, such as skin cancer and premature aging of the skin.
Sunscreen products are known to protect the skin from some of the harmful effects of ultraviolet light exposure. These products contain molecules that absorb the harmful wavelengths of ultraviolet light before they can reach the skin. The absorbed light is converted to heat and rapidly dissipated to the skin and environment, which allows these molecules to revert to a lower energy state, and subsequently absorb another photon of light. In this manner, sunscreen agents can absorb numerous photons of ultraviolet light in a relatively short period of time. By absorbing the harmful wavelengths of light, sunscreen products prevent many of the acute and chronic effects caused by ultraviolet light.
In many countries, sunscreen products are regulated as over-the-counter (OTC) drug products. Most European Economic Community countries and Japan classify these products as cosmetics instead of OTC drugs, however government agencies in these countries also regulate the nature and concentration of the specific agents that can be employed in sunscreen products. Additionally, most (if not all) of the regulatory agencies in these countries define the effectiveness of sunscreen products in terms of the labeling.
The effectiveness of sunscreen products is expressed as a Sun Protection Factor (SPF) value. A SPF value is recognized as the ratio of the irradiation time required to elicit a minimum erythemal reaction (sunburn) on sunscreen protected skin using a solar simulator, to the irradiation time required to elicit the same minimum erythemal reaction (sunburn) on unprotected skin. This test is conducted under clinical conditions according to the procedure described in the Proposed Monograph for Sunscreen Containing Drug Products (hereafter referred to as the Proposed Monograph) published by the U.S. Food and Drug Administration (FDA) in the U.S. Federal Register, Vol. 43, Aug. 25, 1978, Part 2, pages 38206-38269, which is incorporated by reference. As used herein, the term "SPF" or Sun Protection Factor is defined in accordance with the definitions in the Proposed Monograph. This same publication also describes the clinical testing procedure mandated for determining whether sunscreen products are waterproof, water resistant and sweatproof.
The labeled SPF values are generally recognized as being between 2 and 50. This is not meant to imply that SPF values greater than 50 are unachievable given the previous formulation technology. However, the amounts of sunscreen agents needed to achieve such high SPF values are usually cost prohibitive given current formulation technologies. The concentration of sunscreen agents needed to satisfy a `waterproof` designation are particularly high, because some of the agents are washed away in the test that measures SPF for a waterproof composition.
A waterproof product is one that exhibits its labeled SPF value after 80 minutes of exposure to water under conditions that simulate swimming for that period of time. A water resistant product is similarly defined, except that it must withstand 40 minutes of water exposure. Although there is a separate test for the sweatproof claim, the Proposed Monograph allows products that pass the waterproof or water resisitant claim to also carry the sweatproof claim.
Many suncare products also claim to help prevent skin cancer and premature aging of the skin, based upon the wording approved in the Proposed Monograph. Claims are also made about "broadspectrum" protection against both UVA and UVB radiation. These designations are related to the nature of the sunscreens used in a product formulation. The UVB wavelengths of the solar ultraviolet spectrum (290 to 320 nm) are more efficient at producing a sunburn but do not penetrate the skin very deeply. The UVA wavelengths of the solar ultraviolet spectrum are at least 10 times less effective at producing a sunburn, but readily penetrate the entire depth of the skin where a different kind of damage can occur. In higher SPF products, it is usually important to incorporate both UVB and UVA sunscreen agents into the formulation, in which case the formulation is designated as a broadspectrum sunscreen.
The most common suncare products sold in today's market are oil-in-water emulsions incorporating stearic acid neutralized with triethanolaminc. The SPF values of such emulsions range from 2 to 50, and they commonly include ethylhexyl methoxycinnamate as the sunscreen agent. As the SPF of these formulations increases, they commonly contain ethylhexyl salicylate, homosalate, octocrylene and/or oxybenzone in addition to the ethylhexyl methoxycinnamate mentioned above. Alternatively, padimate O can be used in place of the ethylhexyl methoxycinnamate or the salicylates mentioned above. Dioxybenzone, avobenzone or menthyl anthranilate can be used in place of oxybenzone. If the product does not claim to be substantive to the skin (i.e., waterproof or water resistant), trolamine salicylate or DEA methoxycinnamate can be used in place of (or in combination with) the ethylhexyl methoxycinnamate, ethylhexyl salicylate or homosalate. Additionally, sulisobenzone may be used in such non-substantive formulations in place of oxybenzone. The Proposed Monograph lists 21 active ingredients that can be used individually or in combinations to achieve the desired product SPF.
In addition to emulsion (lotion and/or cream) formulations, suncare products can be found in almost any desired form, such as oils, sticks, gels, ointments and pastes. The SPF of these product forms are dependent upon the sunscreen agents employed, their concentration in the formulation, and the content as well as type and amount of any volatile components in the formulation (such as water, alcohol, and volatile oils).
The most popular sunscreen products sold in the market today are TEA stearate based oil-in-water lotion formulations exhibiting SPFs of 15 and above. Most of the SPF 15 formulations contain approximately 7.5% ethylhexyl methoxycinnamate and 4.0% oxybenzone. By judiciously modifying these SPF 15 formulations, and adding approximately 5.0% octyl salicylate to the sunscreen mixture, the SPF values can be increased to a value of about 30. By further modification, including the addition of 8.0% octocrylene, SPF values of up to 50 can be obtained.
An emulsifier technology from Goldschmidt Chemical Company uses a silicone emulsifier which can be used to formulate water-in-oil lotion products. An example of this emulsifier is Abil EM 90, which is a nonvolatile silicone oil that includes cetyl dimethicone copolyol. Another example of this emulsifier is Abil WE 09, which is a nonvolatile silicone oil that contains polyglyceryl-4-isosterate, cetyl dimethicone copolyol and hexyl laurate. The chemical structures of such emulsifiers are disclosed in U.S. Pat. No. 5,482,714, which is incorporated by reference, and which describes use of the emulsifier in a skin protectant emulsion.
U.S. Pat. No. 5,447,715 discloses that volatile silicone oils can be used to improve the SPF value of a non-aqueous waterproof sunscreen composition, but such a non-aqueous product would be unsuitable to formulate aqueous emulsions.
In addition to sunscreen agents, some suncare products contain vitamins and other ingredients. Vitamin E, for example, has been included in suncare products because of its reported benefits in the treatment of sun burns, as well as for its moisturizing properties. More recently, with the advances in scientific knowledge about the effects of ultraviolet light in generating free radicals in and on the skin, this vitamin has become even more important because Vitamin E has been reported to have antioxidant properties. Antioxidants are materials capable of blocking the biochemical cascade of inflammatory mediators produced by free radicals.
In addition to Vitamin E, some suncare product formulators have incorporated Vitamin C (ascorbic acid) or superoxide dismutase into sunscreen products to take advantage of their reported antioxidant properties. It is well known that ultraviolet light has the capacity to create singlet oxygen from normal atmospheric oxygen (also known as triplet oxygen). Since molecular oxygen contains two unpaired electrons in its normal state, the effect of ultraviolet light upon triplet oxygen results in the creation of a more reactive oxygen species (i.e., singlet oxygen). In the singlet state, the oxygen molecule is capable of reacting with a variety of molecules that it would not react with in its normal triplet state. Singlet oxygen can abstract a hydrogen atom from many different molecules creating other free radicals in addition to the superoxide radical from the oxygen molecule. Both of these radicals are extremely reactive with a variety of molecules naturally present in and on the skin. Similarly, singlet oxygen can react with molecules containing double bonds to form peroxy radicals. These radicals can decompose to form hydroxy radicals, which are also extremely reactive in the presence of molecules in and on the skin.
Free radicals produced by the effect of ultraviolet light upon molecular oxygen can be very detrimental to the skin. These free radicals can dramatically increase the permeability of skin cell membranes and lead to cell death, as seen in sunburn cells commonly found in the skin after ultraviolet light exposure. Additionally, if these free radicals are present inside the cells, they can alter proteins, such as enzymes, rendering them ineffective against their known substrates. Free radicals can also alter RNA, thereby disrupting protein synthesis, and damage to the DNA itself can ultimately lead to cancer. Numerous other reactions can be initiated by free radicals inside cells, which can ultimately cause cell death. Additionally, free radicals have been implicated in the effect that ultraviolet light is known to have upon the elastic tissues of the skin, leading to cross linking of collagen and elastic, and sagging, wrinkling and premature aging.
It is therefore an object of this invention to provide an improved skin care composition that is capable of diminishing some of the ill effects of ultraviolet radiation on the skin.
In some embodiments of the invention, the composition also provides more protection from the adverse effects of ultraviolet light, without having to include excessive concentrations of sunscreen agents in a protective formulation.